DE4413127C1 - Process for the production of porous, flowable shaped bodies made of silicon carbide and shaped bodies - Google Patents

Abstract

A process is disclosed for producing porous, current-conducting moulded bodies made of silicon carbide. For that purpose, a granulate with grains in a size range from 0.2 to 10 mm or a powder mixture are used. To form the granulate, silicon, carbon and/or alpha -silicon carbide are used as starting materials, besides a coking organic binder. A green compact is formed therewith and coked in an inert gas atmosphere or in a vacuum by heating up to a temperature in a range from 600 to 1000 DEG C. The green compact obtained by coking is heated up to a temperature from 1600 to 2000 DEG C, causing silicon to react with carbon and form beta -silicon carbide. Components may thus be produced with more or less sharply defined partial regions, layers or zones, whose arrangement in the component determines its temperature profile when current flows therethrough. These areas have a lower or higher specific resistance, as required. To form these moulded bodies, the powder mixtures and/or granulates used to form the various layers and/or regions are composed of coking organic binder, and in addition either of silicon, of silicon and carbon, of silicon, carbon and alpha -silicon carbide, or only of alpha -silicon carbide, and of further doping substances required to achieve the desired electric conductivity.

Description

The invention relates to a method for manufacturing position of porous, flowable molded body made of sili cium carbide, in which a granular fraction is made from granules the grain size range from 0.2-10 mm or from mixed powder, in their formation as starting materials except a carbonizable organic binder or silicon, Carbon and / or α-silicon carbide used a green body is formed, which in a Inert gas atmosphere or in a vacuum by heating coked a temperature in the range of 600-1000 ° C is and in which emerged after the coking process the shaped body to a temperature of 1600-2000 ° C. is heated, the silicon with Carbon converts to β-silicon carbide. It relates furthermore on a flowable shaped body Silicon carbide, electrically heatable via electrical Contacts, with a porosity of 35-75%.

So that a ceramic component at a given Operating voltage and operating temperature as opposites auxiliary heating element can be used either its total electrical resistance is its geometric Shape can be adjusted accordingly or this un ter taking into account the specific electrical Wi  the state of the material.

For example, if it is a highly porous one SiC material as in the case of a flow heating element (DFH) or diesel soot filter, the component must be due the low strength of the material a relative have a thick wall so that the component is both mechanical as well as handling is sufficiently stable. From DE-OS 41 30 630 is a such diesel soot filter known.

Such components require, since they have a uniform Chen have specific electrical resistance and because the entire mass of the component will be heated must, a high expenditure of energy and time to Errei operating temperature. For the application of a such a tubular component as a diesel soot filter with periodic regeneration this fact works particularly disadvantageous because, on the one hand, the one occupied by the soot outer surface of the filter during regeneration against the cooling effect of the incoming engine exhaust only slowly the ignition temperature of the deposited soot is sufficient and, on the other hand, the filtered exhaust gas is unnecessary overheated because the internal temperature of the filter is far higher lies here. A reversal of this temperature profile would significantly reduce these disadvantages.

It is therefore an object of the invention to provide a method create the manufacture of components of a gangs referred to allows more or less sharply delimited sub-areas, layers or zones have, their arrangement in the component whose tempera Determine the door profile when passing electricity. The areas Depending on the requirements - regardless of their bottom rosity - a lower or higher specific  Exhibit resistance.

This object is achieved according to the invention by Ver driving measures solved according to claim 1.

Subareas or layers with different spe specific electrical resistance can be there alone are formed by that - without dopants are used - different mixed powders or Granules used for the individual sub-areas become. For example, you get a partial area with relatively high electrical resistance if one Mixed powder from little binder, to this in stoichiome tric ratio silicon and as much α-Sili cium carbide is used. Depending on what can be achieved specific electrical resistance is therefore assumed the appropriate starting powder or granulate out.

An additional way of training in the outward different electrical properties Chen areas or layers are obtained by the one set of dopants. The effect of the doping fabrics with regard to the desired electrical Properties is basically the origin bound by α-silicon carbide.

The dopants become the starting materials for the Mixed powder or granules added in a suitable amount mixes. Both electrons can be used as dopants donors like antimony phosphorus and tin as well Electron receptors such as aluminum, beryllium and boron elementary or chemically bound. The sub-areas of the moldings from these so predo materials after thermal treatment  according to their type of doping, the type and Amount of additives and - as mentioned above - dependent of the starting materials, different specific electrical resistances.

Should a relatively low specific electrical Wi If the status is reached, then the speaking sub-area provided mixed powder and / or granules less than 70% α-silicon carbide to use. In this case, we also recommend that heating the ent after the coking process standing body under nitrogen and / or stick atmosphere.

If the siliconization is carried out under argon, so who for materials made with beryllium, boron and alumi nium doped, high resistance values for room temperature achieved temperature (<10³ Ohm × cm), while z. B. the Wi the value of Sb-doped material below 10 Ohm × cm lies. In contrast, the siliconization under nitrogen or nitrogenous argon, one finds additional N-doping of the material instead, whereby the above resistance values up to some Powers of 10 can be reduced.

It is advantageous if to form a layer and / or an area with the best possible electri shear conductivity of the nitrogen content in the sinter atmosphere is at least 20%.

In order to obtain a shaped body that is used as a filter it is recommended to contact the manufacturer the mixed powder or granulate after one of the DE-OS 41 30 630 known procedure to proceed. When using this procedure, the fillers  grains coated with the binder and thereby a so-called CM-Ma receive material (CM = Coat Mix):

To produce the mixed powder, first of all a liquid, e.g. B. alcohol, the binder that in the Liquid is completely or partially soluble, and the white tere components of the mixed powder a suspension forms. This is then put into another liquid, with which the solvent is miscible, in which the binder but is not soluble or only sparingly soluble, whereupon the grains coated with the binder as settle muddy mass. The grains are through Decanting removes the excess liquid and then dried.

It is recommended to mix the slurry in a blender mer into the separating liquid by means of a mixing nozzle inject so that the grains with the binder be evenly coated.

The proportion of the binder in the sludge used should be 5 to 30% by weight of the dry weight.

For the mixed powders are starting powders with grain sizes in the range of 0.5-50 µm used. At the Mixing process agglomerate these down to 200 µm.

Different predoped mixed powders or granules can form a shaped body in layers or zones processed after the siliconization layers or zones with different resistance values points. When applying an electrical voltage thus targeted temperature fields within such a gen shaped body are generated.  

To form a shaped body with at least one partially covering this surface layer the surface layer by spraying one for it provided mixed powder slurry on the greens per formed.

The object underlying the invention thus becomes through a flowable molded body of the beginning resolved type, which is characterized by Layers and / or sub-areas with different specific electrical resistance, which is characterized by Doping elements, in particular their type and quantity, differentiate. These layers can also under have different porosities.

Should a shift, for example, in low voltage heated to an operating temperature of 700 ° C are, are - when using the invention Process geometry (thickness and length of the layer) and the specific electrical resistance accordingly to choose.

In the event that the flowable body as a filter element, for example as a diesel soot filter, for one set should come and right over the electrical contacts is to be generated, in particular a flow suitable body with a surface layer points that be opposite to that of the surface layer covered part of the body a higher specific elek has tric conductivity.

The specific electrical resistance of the surface layer of this filter body is 10 ^-2 up to 10 Ohm × cm.

If the flowable body is tubular, it can for example, used as a diesel particulate filter in the way be placed on the outside with which the soot particles entrained exhaust gas from a diesel engine becomes. These particles penetrate when selected accordingly Porosity only about 0.5-2 mm in the surface of the Filter body. To regenerate the filter, i.e. H. for burning the soot collected by the filter body particle, it is therefore sufficient to remove only one of these thicknesses speaking surface layer to heat up. The fat the surface layer and its specific electri shear resistance is therefore chosen so that just the Area of the filter into which the soot particles penetrate, can be heated. In other words: the surface layer has a maximum thickness of 2 mm.

The thickness of the surface layer can be kept small become, if the surface layer is simultaneously fine-pored is so that the soot particles are stopped in it. To get a high gas permeability of the filter ten, the section of the filter that is affected by the Surface layer is covered, chosen coarse-pored.

A practical further development of the throughflow Ren molded body as a diesel soot filter is that the wall of the body at both ends in the area of the attached to the electrical heatability contacts one against at least one between continuous partial area lying on the contacts has higher specific electrical conductivity. Undesirable heating of the ends of the body rend during the regeneration phase (when the current flows) avoided by. Rather, only the thin one Surface layer on the for burning the soot par heated to the required temperature.  

Shaped bodies are shown in the drawing Relation to the executions described below tion examples are explained in more detail. There are further Experimental diagrams reproduced, which are also next be explained below.

Show it:

Fig. 1 molded body with a lying between two non-predoped areas do tated area;

Fig. 2 is shaped body having a planar arrangement of the partial areas;

Fig. 3 longitudinal section through a tubular Fil terelement;

Fig. 4 diagram: specific ohmic resistance of molded bodies or molded body parts depending on different dopings;

Fig. 5 diagram: specific ohmic resistance of molded bodies or molded body parts depending on the nitrogen content of the sintering atmosphere.

Embodiment 1

Production of a molded body with a doped partial region lying between two undoped regions as shown in FIG. 1.

For the production of nitrogen-only doping 200 g of novolak resin were granulated in 1 l of ethanol dissolves and by adding 397 g Si and 70 g electrogra phit (particle diameter of the filler mat used  rialien between 0.5 mm and 50 µm) a slurry manufactured in accordance with the Coat Mix Process (DE-OS 41 30 630) was sprayed with water, where the water-insoluble binder resin on the Filler particles farewell. Filtration made it so coated powder separated from the liquid with agglomerated and dried in a tumble mixer. The fraction of agglomerates with a grain diameter of 0.5-1 mm was separated by sieving.

Another granulate was obtained as described above provides, however, were added to the filler mix 10.7 g of aluminum oxide were added as a predoping agent.

To produce green bodies, an approximately 12 mm wide area was separated by foil in a die with a base area of 15 × 70 mm. On the outside material was filled without predoping and on the inside the aluminum oxide-containing granulate. Fig. 1 illustrates this using a sketch.

After removing the foils, it was pressed by 130 ° C a green body produced.

This green compact was opened by heating under argon Coked at 850 ° C, taking weight loss and dimen changes the binder resin phase in carbon was transferred.

The body was then under nitrogen DE-OS 41 30 630 in a resistance heated oven 1900 ° C heated with the proportions of free coal implemented material and free silicon to silicon carbide without changing the dimensions of the body gene experienced.  

The electrical resistance of the individual zones of the body pers could be determined using the four-point method. It was 0.007 Ωcm at both ends; the one with Al vordo tated phase in the middle of the body showed a specific electrical resistance of 0.014 Ωcm.

Embodiment 2

Production of a molded body with a planar arrangement of the partial areas according to FIG. 2.

According to embodiment 1, two granules were produced provided, the one starting filler mixture as Predoping agent 6.6 g antimony and the other 7.2 g Boron carbide were added.

The granulate fractions were also used 0.2-0.4 mm in the case of the Sb-predoped material, the fraction was used for boron-pre-doped material 1.0-1.6 mm inserted.

A body was produced from these granules with the aid of a molding die according to DE 39 25 844 C2 (base area of the die 15 × 70 mm²), which consisted of a 2 mm thick layer of the granulate predoped with antimony and an approximately 8 mm thick layer of the material Boron predoping existed. The test body was coked as described in Example 1 and siliconized at 1900 ° C under N₂. A sketch of this body is shown in Fig. 2.

The length of the siliconized body was after decrease of a narrow strip 65.3 mm, the width 12.9 mm and the height 9.7 mm. The height of the Sb-doped layer was 2 mm. The total resistance of the body was with  R = 0.182 Q determined.

To determine the resistance of the individual layers the Sb-doped part was removed from the narrow strip separates. The specific resistance of the boron-doped materials could be used with the four-point method ϕ = 0.209 Ωcm can be determined. On the whole body This results in a resistance of 1.378 Ω. The specific resistance of the Sb-vordo arithmetically determined layer with 0.0083 Ωcm become.

Embodiment 3

Production of a molded body with a planar arrangement of the partial areas according to FIG. 2.

According to embodiment 1, after the coat mix Process from 200 g novolak resin, 234 g silicon and 232 g silicon carbide with the addition of 7.2 g boron carbide a starting powder is produced as a pre-doping agent, that was also granulated. From the granulate fracture tion 1.0-1.6 mm of this material was together with the Sb pre-dopant used in embodiment 2 granulate according to a planar material combination Embodiment 2 manufactured, coked and under Silicated N₂ atmosphere.

The specific electrical resistances were for the layer predoped with boron with 0.0083 Ωcm Right.

Embodiment 4

Production of a filter element according to FIG. 3.

The granules used in embodiment 3 were processed into a tubular diesel soot filter, coked and siliconized under a nitrogen atmosphere.

The 1 mm thick "glow and filter layer" as well as the Tube ends made of self-doped material showed one specific electrical resistance of 0.0083 Ωcm, the 5 mm thick boron-pre-doped, containing α-SiC "Support layer" on a 0.520 Ωcm.

Test results according to FIG. 4

In the diagram shown here, the speci cal ohmic resistance of differently doped fer molded body or molded body parts at un different operating temperatures.

These are molded articles that are manufactured according to their not contain more than 30% α-silicon carbide.

The doping is on the respective curve in the diagram element specified. The doping was 1% by weight.

It can be seen from the diagram that a doping with Boron or aluminum leads to a higher resistance, than it has an undoped shaped body.

Doping with antimony leads to less specific electrical resistance.

Another lowering of the specific electrical Resistance was achieved in the case of moldings which nitrogen-containing atmosphere had been sintered. The doping with the above-mentioned elements gives the possibility of different specific electri  set resistances. The lowest spec electrical resistance was in a molded article achieved, the starting material was doped with antimony and which had been sintered in a nitrogen atmosphere.

Test results according to FIG. 5

The diagram shows that there is a nitrogen content of approx. 20% in the sintering atmosphere is sufficient for one specific ohmic resistance as low as possible receive. Lower levels of nitrogen in the Sin teratmosphere result in the possibility of higher speci set ohmic resistances.

The moldings used in the investigation had an α-silicon carbide content after their production of no more than 30%.

Claims (15)

1. Process for the production of porous, flow-through shaped bodies made of silicon carbide, in which a granular fraction from the grain size range of 0.2-10 mm or from mixed powder, in their formation as starting materials, besides a carbonizable organic binder, silicon, carbon and / or α-silicon carbide are used, a green body is formed, which is coked in an inert gas atmosphere or in a vacuum by heating to a temperature in the range from 600-1000 ° C. and in which the shaped body formed after the coking process is heated to a temperature of 1600- 2000 ° C is heated, the silicon reacting with carbon to form α-silicon carbide, characterized in that to achieve a shaped body consisting of layers and / or partial areas of different specific electrical conductivity when forming the green body for corresponding layers and / or areas of mixed powder and / or granules are used which a us the coking organic binder and additionally optionally made of silicon or silicon and carbon or silicon and carbon and α-silicon carbide or only α-silicon carbide to achieve the desired electrical conductivity. 2. The method according to claim 1, characterized, that the mixed powder and / or granule used late in addition with dopants to educate target electrical conductivity have been formed. 3. The method according to claim 1 or 2, characterized, that as dopants electron donors, such as Antimony, phosphorus and tin can be used. 4. The method according to claim 1 or 2, characterized, that as dopants electron receptors, like aluminum, beryllium and boron the.   5. Method according to one of the preceding Expectations, characterized, that in the mixed powder or granulate less than 70% α-silicon carbide is used. 6. The method according to claim 5, characterized, that the heating up after the coking process resulting body under nitrogen or nitrogenous atmosphere. 7. The method according to claim 6, featured, that to form a layer and / or a Be rich with the best possible electrical guidelines  the nitrogen content in the sinterate atmosphere is at least 20%. 8. Method according to one of the preceding Expectations, characterized, that to form a shaped body with a we at least partially this covering surface surface layer by spraying hen a designated mixed powder or Granulate slurry formed on the green body det. 9. Flowable body made of silicon carbide, elec can be heated up and regenerated electrically contacts, with a porosity of 35-75%, marked by Layers and / or sub-areas with difference specific electrical resistance, the themselves by doping elements, in particular their Type and quantity. 10. flowable body according to claim 9, characterized, that layers and / or sub-areas differed have porosity. 11. Flowable body according to claim 9 or 10, marked by a surface layer that is opposite to that of the surface layer covered part of the body a higher specific electrical conductivity having. 12. Flowable body according to claim 11, characterized in that the specific electrical resistance of the surface layer 10 ^-2 up to 10 ohms × cm carries. 13. Flowable body according to claim 11 or 12, characterized, that the surface layer is fine-pored, that of it Sub-area of the molded article with large pores is. 14. Flowable body according to one of the claims 9 to 12, characterized, that the body is tubular. 15. Flowable body according to one of the claims 11 to 14, characterized, that the wall of the body at both ends in Range of electrical heatability brought electrical contacts one opposite at least one between the contacts, continuous sub-area a higher specific has electrical conductivity.